1. Field of the Invention
The present invention relates to angular velocity calculating devices, offset determination methods for the same, and vehicle stopping detecting devices. In particular, the present invention relates to an angular velocity calculating device, an offset determination method for the same, and a vehicle stopping detecting device for averaging angular velocity signals output from a gyroscope (gyro) while a vehicle is stationary to calculate an offset value, and canceling the offset value from an angular velocity signal output while the vehicle is moving to output angular velocity.
2. Description of the Related Art
Angular velocity calculating devices for calculating a rotation angle of a vehicle using signals corresponding to the angular velocity output from a gyro installed on the vehicle are known. In such angular velocity calculating device, as shown in FIG. 12A, the level of the signal output from the gyro includes an offset voltage (Ωoff) such that the level of the signal output from the gyro is not equal to zero even when the vehicle is stationary. For this reason, even when the vehicle is stationary, a calculated rotation angle does not become zero, but increases as shown in FIG. 12B. Accordingly, an angular velocity calculating device for averaging angular velocity signals output from a gyro while a vehicle is stationary may calculate an offset value and cancel the offset value from an angular velocity signal that is output while the vehicle is moving to output an angular velocity as suggested in Japanese Unexamined Patent Application Publication No. 5-306936. In such an angular velocity calculating device, it is necessary to accurately measure the offset value when the vehicle is stationary. In addition, generally, a vehicle speed sensor for generating a pulse for every predetermined travel distance must be installed on the vehicle. Whether or not the vehicle is stationary is then detected on the basis of an interval between the generated vehicle speed pulses.
However, there are cases where an offset value cannot be accurately measured when a vehicle is stationary such as when a vehicle starts to move while a steering wheel is turned. When the vehicle starts to move while the steering wheel is turned, the vehicle travels about 40 to 80 centimeters until the movement of the vehicle is detected as the first vehicle speed pulse P is generated as shown in FIG. 13A. During this time, as shown in FIG. 13B, small angular velocity Ω is output. In a known technique, a wrong offset value is fetched as the point of zero using the angular velocity. Accordingly, a method for averaging gyro output signals at predetermined time intervals to calculate offset values (see, black dots) when a vehicle is stationary, and fetching an offset B calculated a predetermined time Ta before a time point of detection of the movement of the vehicle instead of an offset A calculated immediately before the time point has been suggested in Japanese Unexamined Patent Application Publication No. 5-52578. According to this second known method, an accurate offset value can be obtained.
However, in the second known method, a wrong offset value may be undesirably fetched when the vehicle travels at a low speed, or fetching of a wrong offset value while a turntable on which the vehicle is placed is turning cannot be prevented.
If a vehicle turns gradually at a low speed, a gyro outputs a weak angular velocity signal. However, in a vehicle using a vehicle speed sensor having an insensible zone at a low-speed area, the vehicle speed sensor does not generate vehicle speed pulses when the vehicle travels at a low speed of 1 to 2 km/h. Accordingly, an angular velocity calculating device determines that the vehicle is stationary and calculates an offset value using the weak angular velocity signals. For this reason, since the angular velocity calculating device performs measurement of the direction of the vehicle using the wrong offset value while the vehicle is traveling, accurate direction measurement cannot be made. For example, as shown in FIG. 14, there is a case where a vehicle travels at a low speed after the speed slightly increases temporarily, and the vehicle speed pulse is generated at the time the vehicle starts to move. In such a case, it is impossible to accurately measure the direction using the second known technique.
In addition, in a parking lot equipped with a turntable, since angular velocity is stabilized at a constant level for several seconds, if a turntable smoothly turns as shown in FIG. 15, an angular velocity calculating device determines that a vehicle is in a stationary state and calculates a wrong offset value, which prevents the accurate measurement of the direction.
Additionally, the second known technique is designed to detect the stopping and starting of movement of the vehicle using a vehicle speed sensor. Thus, the second known technique cannot be applied to vehicles not equipped with vehicle speed sensors, and vehicles equipped with vehicle speed sensors may not generate vehicle speed pulses at a low speeds. There are many vehicles made in Europe incapable of physically deriving vehicle speed pulses. In addition, even in vehicles capable of detecting the vehicle speed pulses, the vehicle speed pulses are often used in an antilock brake system. Further, there are vehicles incapable of detecting vehicle speed pulses if the vehicles travel at a low speed of 4 to 5 km/h. Accordingly, detecting the stopping and starting of movement of a vehicle on the basis of a noise level of an output signal of a gyro, such as in Japanese Unexamined Patent Application Publication No. 2000-65849, and then applying the second known technique is considered. However, in a third known technique, it is impossible to accurately detect the stopping and starting of movement of the vehicle and to obtain an accurate offset value, reasons of which will be described below.
Noises superimposed on gyro output signals while a vehicle is stationary include: (1) gyro noises; (2) noises output from a combination of an engine, a transmission, and a body on the vehicle side; and (3) noises output from a navigation equipment. As shown in FIGS. 16A and 16B, the first gyro noises significant vary for each individual gyro. In addition, occurrence of the second noises differs for each model of vehicle, and also differs depending on whether a gearshift lever is shifted into a neutral state or a drive state. For the above reasons, in the third known technique, as shown in FIG. 17, a variation range on the basis of whether the vehicle is considered to be stationary, i.e., a stationary range between TH1 and TH1, is determined in accordance with the individual piece having the greater noise level and the worst noise state. However, in such a stationary range setting method, a start of movement of a vehicle cannot be detected accurately, and a wrong offset A or B is fetched when the vehicle is slightly turning at the time the vehicle starts to move. As a result, a measurement error of the vehicle direction is caused during movement, and a mismatch is caused in map matching of the navigation control or a circumstance in which a vehicle mark is deviated from a road occurs. When an offset error is 5 mV, the error is equivalent to 0.2 [deg/s]. Thus, an angle error can become 0.2×5×60=60 [deg] for five minutes.